Oral squamous cell carcinoma (OSCC) is a common malignancy worldwide associated with a high mortality rate. Cetuximab, a monoclonal antibody targeting the Epidermal Growth Factor Receptor (EGFR), is the first new FDA-approved therapy for OSCC in 45 years, and the only molecular targeting agent approved for use in this cancer. The tyrosine kinase inhibitor (TKI) erlotinib also targets EGFR and is in clinical development for the treatment of OSCC. Despite ubiquitous EGFR expression in OSCC tumors, both cetuximab and erlotinib are effective in only a subset of OSCC patients. Increased understanding of the mechanisms of resistance to EGFR-targeting agents is needed to identify OSCC patients likely to respond, and develop combinatorial treatment regimens to enhance clinical response. Tumor growth in the setting of EGFR blockade may result from the activation of alternative signaling pathways. I have developed and/or begun to characterize OSCC models of cetuximab or erlotinib resistance. My preliminary data demonstrates increased phosphorylation of HER2 and HER3 in preclinical models of cetuximab resistance. Activation of EGFR by HER2 or HER3 may contribute to cetuximab resistance by activating EGFR in a ligand-independent manner. This will be investigated by selective activation with EGFR and HER3 specific ligands in the presence of cetuximab, as well as through siRNA silencing of HER2 and/or HER3 in models of cetuximab resistance. In contrast, I have found increases in the phosphorylation level of the hepatocyte growth factor receptor, c-Met, in preclinical models of erlotinib resistance. Direct activation of the c-Met signaling pathway may provide a growth and survival advantage essential for TKI resistance. This will be investigated by both pharmacological inhibition and gene silencing of c-Met. Additionally, inductive approaches such as gene expression arrays and/or forward-phase protein arrays will be used to identify the molecules downstream of c-Met that may be critical mediators of TKI resistance. The overall goal of this proposal is to determine if alternative signaling through HER2, HER3 and/or c-Met pathways can be targeted to overcome resistance to EGFR inhibitors in OSCC. Public Health Relevance: The Epidermal Growth Factor Receptor (EGFR) is widely expressed in epithelial malignancies including oral squamous cell carcinoma (OSCC). The FDA approved the EGFR monoclonal antibody cetuximab for the treatment of OSCC in 2006 where it remains the only targeting agent approved for use in this disease. EGFR- specific tyrosine kinase inhibitors are under active investigation in OSCC. Despite ubiquitous EGFR expression in OSCC, only a subset of patients will respond to EGFR inhibitors. Biomarkers of resistance to EGFR- targeting agents in OSCC remain incompletely understood. The identification of pathways that confer resistance to EGFR inhibitors could identify new therapeutic targets, that when inhibited, may enhance clinical responses to EGFR-targeting agents.